1. Technical Field of the Invention
The present invention relates generally to a driving circuit including a power transistor operable to drive a load based on a load control signal pro, tided through, for example, a CPU. The driving circuit includes a C-MOS gate and a protective circuit designed to monitor overheating and overcurrent flow in the power transistor and to restrict an operation thereof so as to protect it from damage caused by the overheating and/or overcurrent flow. More particularly, the invention is directed to an improved circuit arrangement of such a driving circuit which does not refine an additional power terminal for the protective circuit.
2. Background Art
FIG. 6 shows a conventional driving circuit 30 known in the art.
The driving circuit 30 includes a power transistor 31 (n-channel power MOS-FET), an AND circuit 32, and a protective circuit 33 for monitoring overheating or overcurrent flow in the power transistor. The power transistor 31 has a drain connected to a load 40 through a drain terminal D and a source connected to ground through a source terminal S. The protective circuit 33 receives a power supply from a power terminal Vcc and has a grounding terminal connected to ground. The AND circuit 32 provides an output signal to a gate electrode of the power transistor 31 through a resistor R when a load control signal inputted to a gate terminal G and a output signal from the protective circuit 33 indicative of a malfunction of the power transistor caused by overheating and/or overcurrent flow are both inputted simultaneously.
In operation, the AND circuit 32 supplies the load control signal inputted to the gate terminal G to the gate electrode of the power transistor 31 under the condition that a malfunction due to overheating and/or overcurrent flow is not detected by the protective circuit 33. The power transistor 31 is then turned on to allow electric current supplied from a power source (+B) to flow through the load 40 connected to drain terminal D. When the protective circuit 33 detects a malfunction of power transistor 31, the AND circuit 32 prohibits the load control signal from being applied to power transistor 31, thereby turning off power transistor 31 so as to protect it from damage due to the malfunction condition.
The load 40 may include an inductance load such as a relay or solenoid or a lamp load. The driving circuit 30 may be incorporated in, for example, an automotive vehicle, and adapted to electrically operate control valves so as to control various engine operating conditions according to a control signal prodded by an electric control unit of an engine. In this case, the load, control signal inputted to the gate terminal G is usually, as shown in the drawing, provided by a CPU 10 through a C-MOS gate 20 of the electric control unit.
The above driving circuit, as clear from the drawing, requires separate power terminals Vcc and grounding terminal GND for the protective circuit 33 in addition to the gate terminal G, the drain terminal D, and the source terminal S. In other words, compared with a typical bipolar power transistor, the driving circuit additionally requires the power terminal Vcc, the grounding terminal GND, and lead wires therefore. This means that a special circuit board must be designed for such a driving circuit.
In addition, since an electric current on the order of from several amps to several hundreds of milliamps usually flows through the power transistor 31 to drive the load 40, a thick lead wire (e.g., a lead wire having a width of more than 3 mm) must preferably be arranged, particularly, between a drain and a source of the power transistor. However, since a lead wire connected to the power terminal Vcc and the grounding terminal GND extends adjacent the power transistor 31, it is impossible to secure a sufficient width of lead wire around the power transistor. A thick lead wire around the power transistor may be arranged by increasing the distance between the power transistor and the protective circuit, but this decreases packaging density.
Japanese Patent First Publication No. 63-229757 teaches a driving circuit with temperature and overcurrent protective circuits. The protective circuits derive power from an input signal provided to the protective circuits and are grounded to a grounding terminal (i.e., a source) of the power transistor. With this circuit arrangement, it is possible to mount circuit elements at high density and at the same time to connect the desired width of lead wire to the power transistor. Furthermore the whole circuit arrangement is simplified and interchangable with another circuit excluding a protective circuit. This prior art driving circuit, however, encounters the following drawback. Since either the temperature protective circuit or the overcurrent circuit uses an analog signal to detect abnormal behavior of the driving circuit and to restrict a signal from being supplied to a gate of the power transistor, the detection accuracy is low. Particularly, the overcurrent protective circuit is adversely affected by variations in temperature and environmental conditions as well as by manufacturing variations. Therefore, in order to accurately protect the power transistor from overheating or overcurrent flow therethrough, it is necessary to detect occurrence of the overheating and overcurrent flow using logic circuits such as gate circuits, a constant current sources, and comparators. A special power terminal and grounding terminal such as those shown in FIG. 6 are thus required to establish stable operation of the protective circuit.